Such heating appliances consist, in a known way, of a burner to produce hot combustion gases to heat the airflow, whereby the combustion gases are guided through one or more elements that each form a flow channel with an inlet and outlet for these gases, while the airflow to be heated, for example originating from a fan, is guided along the outside of these elements.
Thereby heat transfer occurs from the combustion gases to the airflow, whereby the temperature of the combustion gases decreases from the inlet to the outlet.
With high efficiency heating appliances, often abbreviated to HE heating appliances, the heat exchanger comprises a primary section, which, viewed in the flow direction of the combustion gases, is followed by a secondary section in which the combustion gases are further cooled by the airflow in order to allow the water vapour in the combustion gases to condense, whereby, as is known, energy is released with which the airflow can be heated.
The efficiency that can be achieved with such a heating appliance is higher than with traditional heating appliances without a secondary section for condensing the water vapour.
An example of a HE heating appliance is described in U.S. Pat. No. 5,322,050. In this case the primary and secondary sections of the heat exchanger are formed as separate units, whereby the primary heat exchanger is composed of S-shaped folded tubes, while the secondary heat exchanger is a ‘finned’ heat exchanger that consists of tubes on which fins are affixed to increase the heat exchanging surface. Both heat exchangers are connected together by a collector.
A disadvantage of such a heat exchanger is that its assembly is relatively complex and consists of relatively many constituent components.
In U.S. Pat. No. 5,439,050 and U.S. Pat. No. 6,006,741, the primary and secondary heat exchangers are connected together by means of a collector.
A disadvantage of such heat exchangers is that the assembly is difficult to optimise. The primary and secondary heat exchangers must first be manufactured individually and then connected together by means of a separate collector (40 in U.S. Pat. No. 5,439,050).
This collector is relatively expensive as it is generally manufactured from stainless steel.